1. Field of the Invention
The present invention relates to an optical recording medium containing a photochromic material.
2. Description of the Background Art
In recent years, study has been widely made for applying a photochromic material to a recording layer of a recording medium.
When a photochromic material is irradiated with light of a prescribed wavelength, structures of molecules are changed by photochemical reaction and optical properties such as absorbance, optical rotatory power, reflectance and the refractive index are changed in response to the structural change of the molecules.
When the photochromic material is thereafter irradiated with light of a specific wavelength, the changed molecules are returned to the original structures. Therefore, it is possible to record and reproduce information through difference of the optical properties. Further, it is also possible to erase the information by returning the molecules to the original structures.
For example, Japanese Magazine "Bull. Chem. Soc. Jpn" 1990, Vol. 163, pp. 1311 to 1315 discloses a 2,3-bis(2-methyl-benzo[b]thiophene-3-yl) maleic anhydride as such a photochromic material. When light of about 430 nm in wavelength is applied, for example, this photochromic material is changed to such a photostationary state that molecules of ring-opening and ring-closing states are mixed with each other, and colored red. When light of about 550 nm in wavelength is applied, on the other hand, the photochromic material is fully changed to a ring-opening state.
Thus, such reversibly changed two states can be made to correspond to recorded and erased states respectively. In order to read out recorded information, light having a specific wavelength of 550 nm, for example, may be applied to this photochromic material for detecting difference of optical properties, such as absorbance, between the two states.
However, when the photostationary state of such a photochromic material is used as a recorded state and the ring-opening state is used as an erased state, for example, the photochromic material enters a ring-opening state and recorded information is destroyed if reproducing light of about 550 nm in wavelength is applied thereto. When reproducing light of about 430 nm in wavelength is applied, on the other hand, a portion of an unrecorded state (erased state) of the optical recording medium enters a photostationary state (recorded state) to allow erroneous recording, leading to destruction of original information.
Also when the recorded and erased states are reversed, information is destroyed in reproduction.
In relation to such a recording medium, a method of recording information not at only two levels of recording bits of "0" and "1" but at multiple levels is known. For example, Japanese Patent Laying-Open Nos. 61-211835 (1986), 62-164590 (1987) and 1-182846 (1989) disclose such multilevel recording methods.
In each of such conventional multilevel recording methods, photochemical reaction is facilitated in proportion to the amount of photons (amount of irradiation of light) to change absorbance etc. in proportion to such facilitation of the reaction. The amount of irradiation of light is gradually set in order to record information at multiple levels.
However, when optical properties such as light absorption of an optical recording medium containing such a photochromic material are detected by reproducing light for reproducing information, structures of molecules are changed by the reproducing light to destroy the original information. Thus, it is also difficult to reproduce information recorded at multiple levels, similarly to the above.
When information is recorded or reproduced in or from an optical recording medium containing such a photochromic material, information recording density depends on a spot diameter D of light which is used for recording or reproduction. With respect to a wavelength .lambda. of the light and a numerical aperture NA of an objective lens, the spot diameter D has a relation expressed as D.varies.X/NA. Therefore, it is necessary to increase the numerical aperture NA for reducing the spot diameter D. In order to increase the numerical aperture, however, it is also necessary to increase the size of the objective lens, and hence the apparatus is increased in size and the cost therefor is remarkably increased.
Further, an optical recording medium employing such a photochromic material is desired to have sensitivity to a band of longer wavelengths.
In addition, such a photochromic material generates a by-product upon irradiation with light. Thus, the photochromic material is gradually deteriorated when the same is repeatedly subjected to recording and erasing, to finally cause no reaction. This is conceivably because singlet oxygen is formed by irradiation of light, to oxidize/deteriorate the photochromic material.
It has been reported that, among various photochromic materials, a diarylethene derivative exhibits excellent repetition durability, while a 2,3-bis(2-methyl-benzo[b]thiophene-3-yl) maleic anhydride exhibits durability against repetition of 10.sup.3 to 10.sup.4 times.
However, repetition durability of such order is still insufficient in practice.